The Sudbury-based Deep Mining Research Consortium (DMRC) has commissioned a study to investigate the application of geothermal energy technologies to cool deep mine workings and use the heat from underground to produce energy, heat surface buildings and possibly grow vegetables in greenhouses.

“At 10,000 feet, the rock temperature might be in excess of 55 degrees Celsius,” said Mory Ghomshei, an adjunct professor at the University of British Colombia’s Centre for Environmental Research in Minerals, Metals and Materials. “At those depths, you need a lot of cooling, and cooling is very, very expensive. It’s more expensive than heating. That’s why iced cappuccino is more expensive than cappuccino.

“A deep mine could consume 100 megawatts just for cooling, and 100 megawatts is enormous,” he said. “At the same time, there’s demand for heat on surface, so if you bring up the heat, you’re killing two birds with one shot.”

The DMRC is an industry-led research consortium whose members include Vale Inco, Xstrata, Rio Tinto, Goldcorp, Agnico-Eagle, Barrick Gold, CANMET and the City of Greater Sudbury.

Ghomshei is proposing the use of heat pumps and water-to-air heat exchangers at depth to chill mine workings.

The heat pumps would function like a refrigerator, taking heat from one area and moving it elsewhere. The idea would be to extract heat from naturally occurring ground water and pass the chilled water through a heat exchanger to cool the air.

The heated water would then be pumped to surface and used to heat surface facilities.

According to Ghomshei, local cooling in mine workings would be much less expensive than using surface infrastructure to cool and move massive volumes of air underground. Cooling from surface is also complicated in winter, when air and water are too cold and must first be warmed to avoid freezing and plugging up the surface ventilation facility.

District heating

Ghomshei is also looking at the possibility of using geothermal energy from decommissioned mines for district heating.

There are three requirements for bringing geothermal energy to surface: water, heat and permeability, he explained. A decommissioned mine with flooded mine workings has all three. The water circulates in the rock, absorbs heat and is then pumped to surface.

The technology has been successfully demonstrated in Spring Hill, Nova Scotia, where geothermal energy from a decommissioned coal mine is used to heat an industrial park. And Ghomshei is currently working on a feasibility study for the city of Yellowknife in the Northwest Territories to produce up to 10 megawatts of heat from the Con Mine, a gold mine within city limits that was decommissioned in 2000.

The energy produced would be sufficient to heat half of Yellowknife, he said.

Retrofitting homes and other buildings to use geothermal energy as a source of heat could be expensive, he admits, but it could be cost-effective to use geothermal energy for new housing subdivisions or buildings in proximity to a decommissioned mine. An even more interesting application of geothermal energy would be to use it to heat greenhouses, said Ghomshei.

“One greenhouse with 10 acres under glass needs six to seven megawatts of heat, so the idea would be to grow tomatoes and cucumbers locally. When the mine is closed, the community can continue to benefit from it.”

Geothermal energy can also be used to generate electricity, especially around the Pacific Ocean’s Ring of Fire, where underground temperatures are higher and closer to surface. In the Philippines, for example, 20 per cent of electrical power is generated from geothermal power. The West Coast of North America, including British Colombia, is also ideal for geothermal power production, but the technology has been slow to catch on in Canada because electricity from other sources has been so inexpensive, said Ghomshei.

Four kilometers

With a temperature gradient of 25 degrees Celsius per kilometer in Sudbury, it would be necessary to drill four kilometers deep to reach temperatures of 100 degrees Celsius.

Water at a temperature closer to 200 degrees is required to run a turbine, but sufficient pressure can also be produced in locations such as Sudbury by using low boiling temperature material such as isobutene as a working fluid to produce power from geothermal resources at temperatures as low as 70 degrees Celsius.

The so-called enhanced geothermal systems (EGS) technology would require two holes drilled to a depth of four kilometers. The ground between the two holes should be fractured to create an underground geothermal circuit.

“The water would go down one hole, pass through the fractures and return to surface through the second hole,” said Ghomshei.

EGS technology is used successfully to produce power at Los Alamos in the U.S. There are also several ongoing EGS projects in Europe and Australia.

According to Ghomshei, power from an enhanced geothermal system facility might cost 10 to 15 cents per kilowatt/hour, but could be economical with incentives for green technology. A seven or eight megawatt power plant could cost in the neighbourhood of $20 million, he said.